Magnetic data recording devices such as disk drives and tape drives typically have a magnetic medium formatted into multiple data tracks and a magnetic head which must be accurately positioned relative to the data tracks. Typically, each time a drive is powered on the drive must calibrate head position relative to data tracks on the medium. If the magnetic medium is removable then head position must be calibrated relative to tracks on the medium each time the medium is changed. It is common in such devices to have some sort of recorded reference signal on the magnetic medium for head position calibration. For drives with such a reference signal, the drive must move the head to sense the reference signal and determine the boundaries or center line of the reference signal.
An example industry specification for reference signals for data storage tapes is the QIC-80 Development Standard (Revision I, Sep. 2, 1992, available from Quarter-Inch Cartridge Drive Standards, Inc., 311 East Carrillo Street, Santa Barbara, Calif. 93101). This standard provides for a pair of single frequency reference bursts, one on each side of the center line of a tape. The reference bursts are used for head alignment.
Read channels in magnetic data recording devices typically have amplifiers with automatic gain control in which gain is automatically adjusted to hold the amplifier output at some fixed level. If the amplifier used to detect reference bursts has automatic gain control and the input is just noise, gain will automatically increase until the amplifier output level reaches the fixed level. Likewise, if the input is a reference signal, the gain will automatically adjust to hold the output level to the same fixed level. Therefore, with automatic gain control, there is no amplitude discrimination between signal and noise. For head position calibration, any automatic gain control must be disabled and the gain needs to be fixed at a level which discriminates between signal and noise.
In addition to variable gain amplifiers, read channels in a magnetic data recording device typically have circuitry which uses variable amplitude thresholds to help distinguish signals from noise. If the initial amplifier gain is too high, or if the thresholds are too low, noise may be interpreted as data. If the initial amplifier gain is too low, or if the thresholds are too high, the reference signal may not be detected. Therefore, both the initial gain and threshold values are important just to ensure that the reference signal can be detected and distinguished from noise. In addition, improper initial gain and threshold values may result in an inaccurate calibration of head position which in turn may cause a poor error rate when reading data.
In general, with normal variation in amplifiers, media and drives, no single fixed combination of open loop gain and threshold value is suitable. A particular problem in commercially available amplifiers is that the characteristic of open loop gain as a function of input control voltage varies from vendor to vendor. Therefore, a single external control voltage will result in different open loop gain for parts from different vendors. Some read channel calibration is needed before searching for the reference burst. The present invention provides a method for initial calibration of the read channel to discriminate the reference signal from noise with enough signal to noise margin to ensure accurate head alignment.
The following discussion provides additional technical background for the present invention. In a typical magnetic data recording device, binary data is recorded along a track in a magnetic medium by alternately magnetizing small areas from one magnetic polarity to the opposite polarity. The data is encoded in the timing of the polarization reversals, not in the polarity of magnetization. The process of reading typically employs a magnetic head which has a voltage output which is proportional to the rate of change of a magnetic field. For data, the rate of change of the magnetic field (and corresponding voltage) is highest at a boundary where the magnetic polarity reverses. Therefore, the data which was encoded in the timing of magnetic reversals during recording is encoded in the timing of signal peaks during reading. Rather than detect the timing of peaks, the voltage signal is typically differentiated so that peaks in the non-differentiated signal become zero crossings in the differentiated signal. Therefore, in the differentiated signal, the data is encoded in the timing of zero crossings.
With noise, there may be transient zero crossings in the differentiated signal which do not correspond to a magnetic polarity reversal. One solution to help distinguish valid signals from noise is to use a dual path detection system. One path uses the original non-differentiated signal and the other path uses the differentiated signal. In the non-differentiated signal path, the voltage peaks are compared to a predetermined voltage threshold using an analog comparator. The comparator output in the non-differentiated path is used to qualify zero crossings in the differentiated path as follows. During the time window that a voltage peak in the non-differentiated path is opposite in polarity to the previous peak and greater in magnitude than the threshold, any zero crossings in the differentiated path are assumed to be valid. If however the peak voltage in the non-differentiated path is of the same polarity as the previous peak or has a magnitude below the threshold, any zero crossings in the differentiated path during that time are rejected as noise.
The comparator in the non-differentiated path has hysteresis. The comparator has an external hysteresis input for controlling the amount of hysteresis. The comparator hysteresis provides two thresholds, one for each polarity of peaks. If a peak exceeds one threshold, the hysteresis switches the threshold to the opposite polarity so that only an opposite polarity peak can toggle the comparator output.
There is also a peak detector for the non-differentiated path. In typical data reading operation, a fraction of the peak detector output is used to control the hysteresis of the comparator. Therefore, read thresholds are a fixed percentage of signal peak levels.
Circuitry providing dual path detection with qualification as described above is contained within commercially available integrated circuits. For example, the SSI 32P541 Read Data Processor (Silicon Systems Inc., 14351 Myford Road, Tustin, Calif. 92680) contains circuitry for performing the functions described. In addition, there are compatible parts from multiple other vendors. In addition, filters are commercially available which provide both a non-differentiated output and a differentiated output with equal delay for each output. For example, the SSI 32F8130/8131 filters (also from Silicon Systems Inc.) are compatible with the read data processor chips. These filters have an input for external control of bandwidth.